SpudFiles

Larda's hybrid needed to be opened up after each shot to replace the ignition wire. It wasn't destroyed by excessive current density, but by the propellant burn. The glowing wire method is certainly feasible for lower mixes, but at some point the combustion and the gas flow following it will either remove the wire completely, or damage it to the extent that it can't be used reliably again.

Also, the cap method seems excessive. Larda used a tool battery which was connected by a simple switch (can't remember offhand if it was a relay or push-button), with no noticeable ignition delay. If you're going for highly precise ignition timing, you simply don't use a glowing/burning wire method.

How did the propellant burn destroy the wire? Was it from the temperature rise?

How did the propellant burn destroy the wire? Was it from the temperature rise?

I'd guess through the wire being overheated to the point of very low strength and then blown apart and out the barrel by the gas flow. There's a good chance of the wiring igniting as well, with such a high oxidiser density in the chamber.

So there's a possibility it would hold for lower mixes? Like what the rest of use use?

So there's a possibility it would hold for lower mixes? Like what the rest of us use?

DYI wrote:The glowing wire method is certainly feasible for lower mixes, but at some point the combustion and the gas flow following it will either remove the wire completely...

It worked for JSR's clear combustion tests, and it didn't work for Larda's 200X (regrettably not clear) tests. Now that we've got a range to work with, it just needs to be pinpointed

Also, I'd like to point out that "the rest of us", as you refer to them, are just lazy and/or disinterested.

So would loosely encasing the wire in a ceramic tube, with the openings perpendicular to the direction of gas flow offer some protection from the gas flow offer significant protection? I'm certainly able to do that, though I have no hybrid of any sort around right now.

I suspect the big challenge with a glow wire ignition is getting it to survive when the ammo exits the barrel and you get a very high gas flow, followed within a fraction of a second by a reverse rush of air back into the chamber. I suspect that it is actually the reverse flow that is most likely to damage an ignition wire (or a chamber fan).

Combustion itself is a relatively mild mechanical process compared to what happens after the ammo exits the barrel.

I did not know larda's method was not intended to be a one-shot system, but given the results it looks like you need a very short, thick length of nichrome with a HV capacitor dumped though it for the best possible results.

There are charts online that give you the necessary wattage dissipation for given wire gauges and the desired temperature.

It would work without a capacitor, but you'd have to live with pre-heat delay to avoid pushing the maximum temperature too high and burning it out.

Oh, and
GODDAMMIT WHEN AM I GOING TO GET MY PULSE-RATED
HIGH-VOLTAGE SUPERCAPACITORS?!!
Seriously guys, get your sciencing butts in gear so I can finally make a decent electro-something gun

Fnord wrote:It would work without a capacitor, but you'd have to live with pre-heat delay to avoid pushing the maximum temperature too high and burning it out.

Wouldn't a nichrome igniter, like an incandescent light bulb filament, be self limiting? A "conductor" is defined by a positive temperature coefficient of resistance. As the temp goes up so does the resistance. (Semiconductors do the opposite, hence an LED always needs a current limiting resistor since as it heats it draws more current which causes more heating ... and the magic smoke get released.) A nichrome wire should self regulate itself pretty well. I'm pretty sure they do in a toaster plugged into a 120 VAC 15 amp circuit. The toaster won't draw more current than the nichrome wire can handle and the wire itself is responsible for the current limiting.

A typical 100 watt incandescent light bulb has a room temperature resistance of a few ohms or so. In operation the resistance is ~160 ohms.

Jimmy:
I don't know the specs on the resistance of nichrome v. temperature, but you're probably right.
However, it would be better to keep the temperature as low as possible due to mechanical stresses during depressurisation. And keep in mind you then have high-density, 4000F gas adding heat as well.

Another option would be to use a wire coated in a material which would allow it to act as a catalyst. Similar to the thin platinum wire used in windproof lighters. This would reduce the heat necessary to provide ignition.

Of course, platinum is prohibitively expensive but there's surely a way to source very small amounts, or for another element or chemical to function in a similar manner...

Insomniac wrote:platinum is prohibitively expensive

Mildly disagree... the chemistry lab at my high school has several short lengths (3") of thin platinum wire. In addition... I do have some short lengths myself.

Can you solder platinum?

EDIT: Turns out you can't... does anyone know if I can reach the melting point of gold with a propane torch?

Fnord wrote:And keep in mind you then have high-density, 4000F gas adding heat as well.

Not really. Even in a 10x hybrid "high density" is still only about 1% the mass density of most solids and liquids. Therefore, heat transfer to a solid, like the ignition wire, from the combustion gases is pretty minimal.

In a closed combustion chamber, at 1X, the hot gases only heat the chamber walls up by several degrees C. That's nothing compared to the >500C needed to reliable ignite propane in air.

jimmy101 wrote:Wouldn't a nichrome igniter, like an incandescent light bulb filament, be self limiting?

A typical 100 watt incandescent light bulb has a room temperature resistance of a few ohms or so. In operation the resistance is ~160 ohms.

Nichrome wire does have a temperature coefficient that has a wide resistance change with temperature. Due to this it does have some self regulation. The issue is at what point is equilibrium reached? With lower voltage the self temperature regulation is at a lower temperature. This is how light bulbs are able to dim on an autotransformer or light dimmer. The inverse is true too. At higher voltages the equilibrium is reached at a higher temperature. When using capacitor discharge, the equilibrium is very high. The goal is to limit the total joules to the amount required to heat the wire to the desired temperature and then be depleted. Otherwise 300 volts on a wire that will properly regulate at say 6 volts will quickly overheat.

Technician1002 wrote:The goal is to limit the total joules to the amount required to heat the wire to the desired temperature and then be depleted. Otherwise 300 volts on a wire that will properly regulate at say 6 volts will quickly overheat.

Or, don't supply 300V, supply 12V. A 9V battery will heat a strand of steel wool to the point of ignition. Replace that with a suitable gauge of nichrome so it won't actually burn and so that it has a much higher MP.

You are right about the tempcos, iron is about 0.005K^-1, nichrome is roughly 1/10th of that. Still, nichrome is specifically used to make heating elements, including elements that will exceed 1000K (~730C). Here is a handy calculator for nichrome heating elements. For a 9V source, heating to 1200C (!), 4.1 cm long (much longer than needed), you need 40G nichrome wire drawing less than 1 amp. Or, using a 1.5V battery, 1200C, 1 cm length wire, 38G, draws 1.2 amp.